1. Field of the Invention
The present invention relates to a device for the characterization of polymer molecules. More specifically, the present invention relates to a solid state device useful for the characterization of polymer molecules as well as a method of making the same.
2. Description of the Related Art
It has recently been announced that the mapping of the human genome has been completed. This historic development will lead to a myriad of developments ranging from the identification of the genetic basis of various diseases to the formulation and fabrication of new drugs and treatment protocols. All of this will only further serve to increase the already high demand for rapid information processing relating to polymer characterization, particularly that of various nucleic acids (i.e., DNA).
Heretofore, the sequencing of nucleic acids has been performed through chemical or enzymatic reactions. This allows for the nucleic acids to be separated into strains having differing lengths. This is generally tedious and laborious work and requires a significant amount of time and effort to complete. Thus, the results from any desired characterization of a particular polymer sequence are usually quite expensive and take a fair amount of time to obtain.
A significant advancement in the characterization of polymer molecules was introduced by Church et al. in U.S. Pat. No. 5,795,782 which issued on Aug. 18, 1998. Church et al. teach a method of causing polymer molecules, and in particular nucleic acids, to pass through an ion channel in an otherwise impermeable organic membrane. The membrane separates two pools of a conductive fluid solution containing a supply of the polymer material in question. By generating a voltage differential across the membrane, the polymer molecules can be ionized or polarized and guided through the ion channel. By measuring the various electrical characteristics of the membrane, the particular base of the polymer molecule can be identified by identifying the changes in these electrical characteristics as a particular base of the polymer molecule occludes the channel. Thus, each type of base member will exhibit unique characteristics that are identifiable by variations in monitored electrical parameters such as voltage or current.
The drawback of this device is that it is difficult to create an impermeable membrane having a sufficiently small ion channel that will allow the device to function properly. Church, et al. teaches using an organic membrane where an ion channel is created through the membrane via a chemical etching process. This is extremely difficult to do on a cost effective and repetitive scale. Specifically, the formation of an otherwise impermeable organic membrane and chemically etching or otherwise forming the ion channel is a hit or miss operation that may or may not actually produce the appropriately channeled membrane. Thus, while the concept of providing for the rapid determination of the character of polymer molecules is an extremely important one, no device has been provided that can be reliably produced while achieving accurate results.
Therefore, there exists a need to provide a high quality, reliable and easily reproducible polymer characterization device.
The present invention provides a generally impermeable membrane having a nano-scale aperture. Polymer molecules are caused to travel through the aperture or channel and the electrical characteristics generated by the particular base or monomer occupying the channel at a given time is determined based upon various measurements made by monitoring the membrane.
In one embodiment, the membrane is used to separate two pools of a conductive medium containing quantities of the polymer molecules in question. Unlike membranes used by the previous device which are organic in nature, the membrane of the present invention is inorganic and uses a combination of wafer and thin film technology to accurately and consistently manufacture a membrane having the desired characteristics. The membrane is formed by providing a base preferably using a silicon substrate. A thin film is deposited on one side of the silicon substrate. The thin film includes one or more integrated electrical leads that can ultimately be connected to the testing and monitoring equipment. Using standard lithography techniques and taking advantage of the anisotropic etching characteristics of single crystal silicon wafing, a micro-scale hole is etched through the silicon substrate. In the selected area, the etching process removes all of the silicon substrate but leaves the thin film entirely intact and unaffected. Thus, a self supporting thin film, such as SiN for example is bridged across a micro-scale aperture in a silicon substrate. Using a focused ion beam or electron beam lithography, a nano-scale aperture is precisely cut through the thin film layer. Thus, the nano-scale aperture provides a channel through which polymer molecules pass and are measured in various ways.
The present invention also provides for differing configurations of the thin film layer. At a minimum, a single electrically conductive layer should be provided. If properly configured, the fabrication of the nano-scale aperture will bisect this conductive layer into two independent and electrically isolated conductive members or leads. Thus, as a molecule passes through the channel, monitoring equipment connected to each of the electrically conductive sections can obtain measurements such as voltage, current, capacitance or the like. This would be a transverse measurement across the channel.
In practice, it may be more practical to provide one or more dielectric layers that effectively protect and insulate the conductive layers. The use of such dielectric layers can simplify the manufacturing process and allows for multi-level conductive layering to be generated. That is, providing a single conductive layer or effectively providing electrical leads in a common plane allows for measurements of the particular polymer base in a transverse direction. However, by stacking conductive layers atop on another (electrically isolated from one another such as by an interposed dielectric layer), measurements of certain electrical characteristics can be taken in the longitudinal direction.
The present invention provides for a variety of lead patterns in both a longitudinal and transverse direction. In one embodiment, a single, shaped electrically conductive layer is provided. The conductive layer is relatively narrow near a medial portion so that a channel formed therethrough by a focused ion beam effectively bisects the electrically conductive layer into two electrically independent sections or leads. The benefit of such a construction is a minimal number of steps are required to complete the finished product. However, one potential drawback is that the single conductive layer must be applied relatively precisely in that the channel which eventually separates the layer in two will usually have a diameter on the order of ten nanometers.
Since this level of precision may be difficult in some manufacturing processes, another single layer approach is provided. Namely, a single electrically conductive layer is provided. However, the medial portion need not be so narrow as to allow bisection by the formation of a nano-scale aperture. Thus, when a nano-scale aperture is bored through the thin film layer, electrically conductive material remains which effectively connects the two leads. A focused ion beam or other precision material removing apparatus is used to remove a section of the thin film layer so that the two leads are electrically independent.
By providing leads on a single plane, various transverse measurements of electrical characteristics can be performed. Bisecting a single layer results in the formation of two leads. The present invention also provides for fabricating four or more leads in a single plane so that multiple transverse measurements are possible.
By utilizing dielectric layers, electrically conductive leads can be fabricated in multiple planes. This not only allows for transverse measurements to be made, but facilitates longitudinal measurements as well. Any configuration or variation of the single plane lead structures can be repeated with the multi-level thin film layers. Namely, relatively precise conductive layers can be applied relying on the focused ion beam or other precision cutting device to bisect each respective layer. Alternatively, a focused ion beam or other precision cutting device can be utilized for removing a precise amount of the electrically conductive layer in and around the desired channel area, once again resulting in any number of leads being fabricated in any given plane. Thus, multiple transverse and multiple longitudinal measurements can be made between any given pair of leads.
Longitudinal measurements in and of themselves may be sufficient to determine the necessary characteristics in the polymer material in question. That is, it is not necessary to have electrically isolated lead pairs in a single plane. This allows for an embodiment where a relatively imprecise electrically conductive layer is formed in a first plane. A second relatively imprecise electrically conductive layer is formed in a second plane wherein the second plane is separated from the first by a dielectric layer. By providing a nano-scale aperture through the entirety of the thin film layer (i.e., the dielectric layers and both the conductive layers), a completed structure is fabricated. In this embodiment, electrical measurements are not possible within a single plane. However, by measuring across different planar levels sufficient information may be gathered to characterize the polymer molecule. This configuration provides for relative ease during the manufacturing process and results in a repeatable and highly accurate device.